Project 3 - Summary Vascular injury and vessel wall diseases serve as triggers for the accumulation of platelets and fibrin, sometimes with disastrous consequences. Platelet activation mechanisms are commonly studied in vitro using tools that work best at the level of individual platelets or small groups of platelets. Here we have made a paradigm shift, viewing both hemostatic and pathologic thrombus formation as the product of large platelet populations and combining observational, experimental and computational approaches to understand the relationships among those populations. Our overall goal is to obtain new insights into platelet activation as it occurs in vivo and identify better ways to limit thrombosis without overly impairing hemostasis. Our premise is that as platelets begin to accumulate at a site of injury, they alter their local microenvironment in ways that affect subsequent events. We and others have shown previously that there are regional differences in the extent of platelet activation within hemostatic thrombi, resulting in a core of fully-activated, closely-packed platelets overlaid by a shell of less-activated platelets. Here we hope to understand how these regional differences arise and influence subsequent thrombus growth and stability. In Aim #1 we will apply novel technologies to determine how regional differences in platelet packing density, intrathrombus solute transport and agonist distribution arise and interact with the platelet signaling network. In Aim #2, we will collaborate with fellow PPG members, Sriram Krishnaswamy and Rodney Camire, to test the hypothesis that thrombus architecture, as well as the location of procoagulant membranes, dictate the distribution of thrombin and fibrin during the hemostatic response. Finally, in Aim #3 we will combine observational and experimental data from Aims #1 and #2 with computational methods to test hypotheses about thrombus formation that cannot be addressed by experimental approaches alone, including the hypothesis that the formation of a core-and-shell architecture is a biological mechanism evolved to limit thrombus growth and prevent vascular occlusion. All three aims are built upon considerable preliminary data and take advantage of new and existing methods to study thrombus formation. They will also take advantage of the collective expertise of a team of investigators with strong backgrounds in platelet biology, mouse models of hemostasis, methods development, and the use of computational and applied engineering approaches to answer biological questions.